Brown Dwarfs Could Be More Common Than We Thought

In 2007, something strange happened to a distant star near the centre of our galaxy; it underwent what is known as a ‘microlensing’ event. This transient brightening didn’t have anything to do with the star itself, it had something to do with what passed in front of it. 1,700 light years away between us and the distant star, a brown dwarf crossed our line of sight with the starlight. Although one would think that the star would have been blocked by the brown dwarf, its light was actually amplified, generating a flash. This flash was created via a space-time phenomenon known as gravitational lensing.

Although lensing isn’t rare in itself (although this particular event is considered the “most extreme” ever observed), the fact that astronomers had the opportunity to witness a brown dwarf causing it means that either they were very lucky, or we have to think about re-writing the stellar physics textbooks…
“By several measures OGLE-2007-BLG-224 was the most extreme microlensing event (EME) ever observed,” says Andrew Gould of Ohio State University in Columbus in a publication released earlier this month, “having a substantially higher magnification, shorter-duration peak, and faster angular speed across the sky than any previous well-observed event.”

OGLE-2007-BLG-224 revealed the passage of a brown dwarf passing in front of a distant star. The gravity of this small “failed star” deflected the starlight path slightly, creating a gravitational lens very briefly. Fortunately there were a number of astronomers prepared for the event and captured the transient flash of starlight as the brown dwarf focused the light for observers here on Earth.

From these observations, Gould and his team of 65 international collaborators managed to calculate some characteristics of the brown dwarf “lens” itself. The brown dwarf has a mass of 0.056 (+/- 0.004) solar masses, with a distance of 525 (+/- 40) parsecs (~1,700 light years) and a transverse velocity of 113 (+/- 21) km/s.

Although getting the chance to see this happen is a noteworthy in itself, the fact that it was a brown dwarf that acted as the lens is extremely rare; so rare in fact, that Gould believes something is awry.

“In this light, we note that two other sets of investigators have concluded that they must have been ‘lucky’ unless old-population brown-dwarfs are more common than generally assumed,” Gould said.

Either serendipity had a huge role to play, or there are far more brown dwarfs out there than we thought. If there are more brown dwarfs, something isn’t right with our understanding of stellar evolution. Brown dwarfs may be a more common feature in our galaxy than we previously calculated…

Precicely how rare is the key to solving this question. I think we need to continue looking for similar events although I suspect brown dwarves are very common and probably form in clusters where local conditions lend themselves to their formation

I’ve long suspected that brown dwarfs and brown sub-dwarfs (makes more sense than sub-brown dwarf) are more common than red dwarfs, which are thought to be the most common stars.

That may account for a lot of the missing mass.”

Gnat, I’ve been saying the same thing, here and elsewhere, for a long time. Everytime our technology and/or techniques improve we find more lots and lots ordinary matter.

I remember 40 years ago when we were astounded by astronomers’ estimates that there were over a hundred million stars in the Milky Way, The current estimates are a couple hundred BILLION, and it’s likely to keep going up.

Estimates of the number of stars even in intergalactic space are rising exponentially as well. I saw that up to 10% of the mass of one huge galactic cluster is now believed to be in stars that have been ripped out galaxies as they collide and we’re only beginning to be able to see them now. If fact, I’d say it’s likely that stars can form in huge numbers even independent of galaxies but our telescopes are just not powerful enough to see them (yet).

When a gas cloud collapses and stars form in it, it is likely that bodies of every size possible form within it as well. It’s likely that there are trillions of non-stellar earth size bodies and up formed without a star in just this galaxy alone.

By the time we’re done finding all the ordinary matter (if ever), the need for “dark matter” to make the equations work may be rendered largely unnecessary.

@ Greg, Roger and Dark Gnat, For many years, MAssive Compact Halo Objects (MACHOs) were extensively searched for in the hopes that this may resolve the question of ‘what is DM and what does it consist of?’ Unfortunately, brown dwarfs, interstellar (unbound) planets, neutron stars, white dwarfs, and Black Holes were considered and found to be lacking in mass and/or number to fully account for the rotation curve of the MWG. In their paper, the authors do not find that extrapolating data from their observations lead to a ‘brown dwarf halo’ around the MWG that accounts for the galaxy rotation curve. However, it would appear than more brown dwarfs (mainly in the disk, btw) do exist than are predicted by theories of brown dwarf formation. Great story, Ian, and thanks for the link to the paper.

the OGLE collaboration surveys the sky and monitors the brightness of thousands of stars at once. If the computers detect a star brightening, at all, an alert is sent out, where the individual object can be followed up by multiple observers. alot of times it ends up being a false alarm, but other times the brightness keeps brightening and a microlensing event is observed.

Doesnt stretch anyone’s credibility at all. check out more info on OGLE, and gravitational microlensing on wikipedia for a more detailed overview, then look though papers/books for details if you want.

@ Timber, The reason this event was so well documented is the results of collaborative observing programs( OGLE, MOA and uFUN) networking to get as many scopes as possible on high value Targets of Opportunity (ToO). In this case, the paper states the OGLE Collaboration issued an alert 20 hours before maximum light that a microlensing event could be taking place( they were seeing the very beginning of this event). In turn, the 2 other collaborations also sent out alerts to participating observatories or made observations. See page 4 of the paper linked to above for details on the scopes involved and page 12 has a global map of the observatories making observations. Good question, though.

@ Dark Gnat, Check out the Wiki page on MACHOs here: http://en.wikipedia.org/wiki/MACHO .Remember, MACHOs include brown dwarfs. The entry states “Theoretical work simultaneously also showed that ancient MACHOs are not likely to account for the large amounts of dark matter now thought to be present in the universe. MACHOs may still constitute 20% of the dark matter in the Milky Way Galaxy.” Remember, too, that brown dwarfs constitute only a portion of objects considered to comprise MACHOs. Particularly check out reference paper 5 for more technical reasons DM is likely nonbaryonic. These new findings, if confirmed, may add a few percent to the total MACHO makeup. But certainly brown dwarfs in the outer halo make some contribution to baryonic DM. It may have to wait for projects like the LSST sky survey to more accurately pin down brown dwarf like number and distribution.